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Summary

We describe a protocol to photogenerate N-heterocyclic carbenes (NHCs) by UV irradiation of a 2-isopropylthioxanthone/imidazolium tetraphenylborate salt system. Methods to characterize the photoreleased NHC and elucidate the photochemical mechanism are proposed. The protocols for ring-opening metathesis photopolymerization in solution and miniemulsion illustrate the potential of this 2-component NHC photogenerating system.

Abstract

We report a method to generate the N-heterocyclic carbene (NHC) 1,3-dimesitylimidazol-2-ylidene (IMes) under UV-irradiation at 365 nm to characterize IMes and determine the corresponding photochemical mechanism. Then, we describe a protocol to perform ring-opening metathesis polymerization (ROMP) in solution and in miniemulsion using this NHC-photogenerating system. To photogenerate IMes, a system comprising 2-isopropylthioxanthone (ITX) as the sensitizer and 1,3-dimesitylimidazolium tetraphenylborate (IMesH+BPh4-) as the protected form of NHC is employed. IMesH+BPh4- can be obtained in a single step by anion exchange between 1,3-dimesitylimidazolium chloride and sodium tetraphenylborate. A real-time steady-state photolysis setup is described, which hints that the photochemical reaction proceeds in two consecutive steps: 1) ITX triplet is photo-reduced by the borate anion and 2) subsequent proton transfer takes place from the imidazolium cation to produce the expected NHC IMes. Two separate characterization protocols are implemented. Firstly, CS2 is added to the reaction media to evidence the photogeneration of NHC through formation of the IMes-CS2 adduct. Secondly, the amount of NHC released in situ is quantified using acid-base titration. The use of this NHC photo-generating system for the ROMP of norbornene is also discussed. In solution, a photopolymerization experiment is conducted by mixing ITX, IMesH+BPh4-, [RuCl2(p-cymene)]2 and norbornene in CH2Cl2, then irradiating the solution in a UV reactor. In a dispersed medium, a monomer miniemulsion is first formed then irradiated inside an annular reactor to produce a stable poly(norbornene) latex.

Introduction

In chemistry, N-heterocyclic carbenes (NHCs) species fulfill the twofold role of ligand and organocatalyst1. In the former case, the introduction of NHCs has resulted in the design of metal transition catalysts with improved activity and stability2. In the latter case, NHCs have proved to be superior catalysts for manifold organic reactions3,4. Despite this versatility, handling bare NHCs is still a significant challenge5, and producing these highly reactive compounds so they are released in situ and "on demand" is a very attractive goal. Consequently, several strategies have been developed to release NHC in the reaction media which mostly rely on the use of thermolabile progenitors6,7,8. Surprisingly, while this could unleash a novel generation of photoinitiated reactions useful for macromolecular synthesis or preparative organic chemistry6, generation using light as stimulus has been scarcely explored. Recently, a first photo-generating system able to produce NHC has been unveiled9. It consists of 2 components: 2-isopropylthioxanthone (ITX) as photosensitive species and 1,3-dimesitylimidazolium tetraphenylborate (IMesH+BPh4-) as the NHC protected form. Consequently, in the following paragraphs, we report a method to generate the NHC 1,3-dimesitylimidazol-2-ylidene (IMes) under UV-irradiation at 365 nm, characterize it, and determine the photochemical mechanism. Then, we describe a protocol to perform ring-opening metathesis polymerization (ROMP) in solution and in miniemulsion using this NHC photogenerating system.

In the first portion, we report a synthesis protocol to produce IMesH+BPh4-. This protocol is based on anion metathesis between the corresponding imidazolium chloride (IMesH+Cl-) and sodium tetraphenylborate (NaBPh4). Then, to demonstrate the in situ formation of NHC, two protocols involving the irradiation at 365 nm of a IMesH+BPh4-/ITX solution in a photoreactor are described. The first consists of monitoring the deprotonation of the imidazolium cation IMesH+ through 1H NMR spectroscopy. Direct evidence for formation of the desired NHC (IMes) is provided in a second method, where the adduct IMes-CS2 is successfully isolated, purified, and characterized.

The second section describes two protocols that shed light on the photochemical mechanism involving the NHC two-component photogenerating system IMesH+BPh4-/ITX. Firstly, an original real-time steady state photolysis experiment reveals that electron transfer is induced by photo-excitation of ITX in the presence of tetraphenylborate. Electron donor properties of this borate anion10 drives a photoreduction of 3ITX* triplet excited-state into ITX●- radical anion through a so-called photo-sensitized reaction. The formation of NHC confirms that ITX●- species may further abstract a proton from IMesH+ to produce the desired NHC. Based on acid/base titration using phenol red pH indicator as titrant, a second original protocol is implemented that allows the determination of the yield of released NHC.

In the third section, we describe a protocol in which the above-mentioned photogenerated IMes can be exploited in photopolymerization. Of primary interest is ring-opening metathesis polymerization (ROMP), because this reaction is still at a preliminary stage of development with regard to photoinitiation11,12. Initially limited to ill-defined and highly sensitive tungsten complexes, photoinduced ROMP (photoROMP) has been extended to more stable complexes based on W, Ru, and Os transition metals. Despite the variety of precatalysts, almost all photoROMP processes rely on the direct excitation of a single photoactive precatalyst13. By contrast, we use radiation to create the NHC imidazolidene ligand (IMes), which can react subsequently with a non-photoactive Ru precatalyst [RuCl2(p-cymene)]2 dimer9. In this method, the photogeneration of NHC ligand drives the in situ formation of a highly active ruthenium-arene NHC complex known as RuCl2(p-cymene)(IMes) (Noels' catalyst)14,15. Using this indirect methodology, two distinct photoROMP experiments of norbornene (Nb) are performed: 1) in solution (dichloromethane) and 2) in aqueous dispersed system from a monomer miniemulsion16.

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Protocol

1. NHC Photogenerating System: Synthesis and Reactivity

Synthesis of 1,3-dimesitylimidazolium tetraphenylborate (IMesH+BPh4-)

Preparation of the solution of 1,3-dimesitylimidazolium chloride (IMesH+Cl-) in ethanol.

Add (dropwise) the solution of sodium tetraphenylborate into the solution of 1,3-dimesitylimidazolium chloride under stirring.

Stir the reaction mixture for 10 min at room temperature.

Remove the stir bar and filter the white precipitate using a vacuum and fritted glass filter of pore size 3.

Wash the precipitate with 30 mL of ethanol and filter it (fritted glass filter with pore size 3). Wash the precipitate with 30 mL of deionized water and filter it (fritted glass filter with pore size 3).

Dry the white precipitate at 60 °C for 15 h. Analyze the product by 1H and 13C NMR in DMSO-d6 according to previously reported procedures9.

Photogeneration of NHC 1,3-dimesitylimidazol-2-ylidene, also known as IMes, by UV irradiation of the dimesitylimidazolium tetraphenylborate in the presence of isopropylthioxanthone (ITX)

Place the NMR tube inside the photochemical reactor equipped with a circular array of 16 fluorescent tubes emitting a monochromatic radiation at 365 nm and irradiate for 10 min.

Monitoring of deprotonation of IMesH+BPh4- by 1H NMR spectroscopy

Analyze the deprotonation of IMesH+ into IMes by 1H NMR.
NOTE: 1H NMR spectra were recorded at 25 °C on a NMR spectrometer operating at 400 MHz. TMS was used as internal standards for calibrating the chemical shifts in 1H NMR.

Calibrate the integration parameters so that in the 1H NMR spectra the CH3 singlet of 1,3-dimesitylimidazolium tetraphenylborate (δ = 2.0 ppm) corresponds to six.

Determine the integration value of the N-CH-N signal area (δ = 8.4-9.4 ppm) in order to evaluate the degree of IMesH+ deprotonation. The integration value should vary from 1 (when no deprotonation occurred, before irradiation) to 0 (when complete deprotonation of IMesH+ has been performed).

Formation, isolation, and characterization of the 1,3-dimesitylimidazoliumdithio-carboxylate adduct (IMes-CS2)

Add 0.02 mL of carbon disulfide in the as-irradiated NMR tube. The reaction media changes in color from orange/brown to dark red, indicating the formation of the IMes-CS2 adduct.

Let it react for 12 h. A red precipitate forms assigned to the IMes-CS2 adduct.

Filter the red precipitate (fritted glass filter with pore size 3) and dry it under air at room temperature for 12 h.

Solubilize the red solid in 0.5 mL of deuterated DMSO. Confirm the chemical structure by 1H and 13C NMR spectroscopy.
CAUTION: Carbon disulfide is highly toxic and should be handled with care under a fume hood.

Transfer 2 mL of this freshly prepared solution into a conventional spectroscopic quartz cell capped with a rubber septum.

Purge the colorless mixture with nitrogen before exposing the cuvette to a 365 nm LED spotlight (power of 65 mW) for 1 min.

After each irradiation time, add gradually 0.1 mL portions of phenol red (PR) solution (2 x 10-4 M in dry acetonitrile) into the cuvette. This latter titrating solution was prepared in advance.

Record a UV-vis spectrum after each 0.1 mL addition of PR solution until reaching 1 mL.
NOTE: The indicator solution is initially transparent and contains the bis-protonated form H2PR. After its addition, acid/base reaction with NHC causes the formation of the pink bivalent anion PR2- with a maximum absorption at 580 nm. Plotting the absorbance at 580 nm as a function of the titrant volume gives two intersecting straight lines, indicative of the titration endpoint.

Repeat the same procedure (steps 2.2.1-2.2.5) with the same ITX/IMesH+BPh4- solution irradiated for longer times: 2 min, 5 min, and 10 min. For each time, a new IMesH+PH4-/ITX sample must be prepared.
NOTE: At the equivalence point in the acid-base titration: (1)
Where is the concentration of photogenerated IMes released in the UV cuvette, V is the initial volume of IMesH+BPh4-/ITX solution, [PR] is the concentration of PR, and Veq is the total volume of PR added into the UV cuvette at the titration end-point. Therefore, the yield of IMes released upon irradiation of IMesH+BPh4-/ITX solution is obtained from equation (2): (2)
Where is the initial concentration of IMesH+BPh4-.
The validity of the method is checked by titrating a free IMes solution (1 x 10-4 M in acetonitrile) using a similar acetonitrile PR solution as a titrant (2 x 10-4 M).

Dissolve the solids in 10 mL of dichloromethane and cap the tube with a rubber septum.

Purge the mixture by bubbling nitrogen gas through a syringe needle for 15 min.

Place the tube inside the photochemical reactor equipped with a circular array of 16 fluorescent lamps (emitting at 365 nm) and irradiate for 10 min. The solution becomes viscous, indicating that high-molecular weight polyNb is formed.

Precipitate the polymer by pouring the solution into 300 mL of methanol.

Filter the polymer (fritted glass filter with pore size 3) and dry it at 60 °C for 8 h.

Analyze the polymer by 1H NMR according to reported procedures9 by dissolving about 10 mg of polymer in 0.5 mL of CD2Cl2.

Analyze the polymer by size exclusion chromatography according to reported procedures9, using THF as eluent and dissolving 10 mg of polymer in 1 mL of THF.

Mix the two organic solutions containing the monomer and the catalytic mixture under a nitrogen flux, and introduce 15 g of the final organic solution inside the photoreactor, containing the aqueous phase under stirring.

Stir the two phases during 1 h to form a rough macroemulsion. Sonicate during 10 min (power 50%; pulse on-time: 5 s, off-time: 5 s) to form the miniemulsion.

Photopolymerization of NB miniemulsion.

Replace the airtight sonication probe by the LED lamp equipped with a water cooling system and protected by a cladding tube under a nitrogen flux.

Place the closed reactor inside the photocabinet to prevent exposure to UV radiation.

Irradiate the monomer miniemulsion for 100 min to obtain polymer latex. During irradiation, particle size and monomer conversion can be determined as explained below.

Precipitate the rest of the sample in 20 mL of acetone. Filter the polymer. Dry the polymer under a vacuum and measure the molecular weight by size exclusion chromatography (SEC) [SEC in tetrahydrofuran (THF) (1 mL min-1) with trichlorobenzene as the flow marker, using both refractometric and UV detectors].
CAUTION (Part 1-3): Possibly hazardous sources of light emitting in the UV and visible range are used in the described experiments. These lamps can present a reasonably foreseeable risk of harming the eyes and skin of lab members. Consequently, all measures possible should be put in place by the experimenter to reduce the risks to as low as is reasonably practicable. A list of common measures includes the isolation of the light source inside a protective casing (photocabinet, for example), training of all workers, placing the hazardous sources of light in well-designated laboratories or fume hoods with restricted access, providing suitable safety gears (safety goggles blocking UVA irradiation are sufficient for all described protocols), and displaying appropriate warning and safety signs.

Step 1.2 describes how to generate the N-HC IMes by irradiating the mixture IMesH+BPh4-/ITX (2/1 equiv.) in THF-d8 solution.

Step 1.3 shows that it is possible to assess the conversion of IMesH+ in IMes by monitoring the deprotonation of IMesH+BPh4- through 1H NMR spectroscopy. Figure 2 shows that proton Ha (8.63 ppm, Figure 2a) on carbon 2 adjacent to the two nitrogen atoms disappears partially after 10 min irradiation (53%, Figure 2b). The reaction was performed by irradiating the mixture IMesH+BPh4-/ITX (2/1 equiv.) in THF-d8 solution.

Step 1.4 shows that it is possible to isolate the formed NHC by reacting the as-irradiated medium (see protocol 1.2) with CS2. The red precipitate formed in THF-d8 is collected, dried, and dissolved in DMSO-d6. As can be seen in the 13C NMR spectrum (Figure 2c), all the characteristic resonances are consistent with IMes-CS2 adduct. This result indirectly confirms the in situ generation of the targeted IMes NHC.

Step 2.1: Thioxanthone derivatives make up a well-established class of photoinitiators generally employed in combination with a second component referred to as "co-initiators". Their absorption spectra appear with a maximum in the range of 340-420 nm. The nature of the co-initiator determines the mechanism of initiation. Three general initiation mechanisms have been described: 1) triplet-triplet energy transfer (in this case, from 3ITX* to 3BPh4-*); 2) electron transfer from the electron donor BPh4- to 3ITX*; and 3) direct H abstraction of IMesH+ by 3ITX*. Mechanism 1 can be discarded since the triplet energy order ET(BPh4-) > ET(ITX) is established by conventional computational procedure.

Step 2.1 provides evidence as to whether mechanism 2 or 3 is operating. Figure 3 shows the evolution of absorbance values of characteristic ITX absorption band at 365 nm during irradiation for three different bicomponent mixtures: IMesH+BPh4-/ITX, IMesH+Cl-/ITX, and NaBPh4/ITX. The absence of decay for IMesH+Cl- supports the incapacity for electronically excited ITX to abstract a hydrogen from the imidazolium cation (mechanism 3). In contrast, photobleaching of ITX is visible in the two systems containing the BPh4- anions; although, the decay rates are different in these two cases. This result emphasizes the critical role played by the tetraphenylborate anion. Consequently, the photoreduction of ITX by tetraphenylborate (mechanism 2) is proven to be the primary step in the formation of the NHC. Figure 4 displays a hypothetical and complete mechanism in which the ITX●- radical anion may abstract a proton from IMesH+ to release the free NHC IMes.

Step 2.2 shows evidence in favor of this mechanism. This method reveals the progressive release of NHC during irradiation. It is a method to determine the amount of released NHC based on acid/base titration using phenol red (PR) pH indicator as titrant. A maximum yield of 50% is achieved after 5 min of irradiation (Figure 5), and a control experiment with free IMes enables validation of the method.

Step 3.1 describes photoROMP of NB (540 equiv.) in dichloromethane using a photolatent mixture composed of IMesH+BPh4-/ITX (10/5 equiv.) (to produce NHC IMes) and the well-known inactive [RuCl2(p-cymene)]2 dimer (1 equiv.). It is recognized that the simple reaction of Ru precatalyst with the imidazolidene ligand IMesis a means to generate in situ the highly active ruthenium-arene complex RuCl2(p-cymene)(NHC), also known as Noels' catalyst. Irradiation is performed in a conventional photochemical reactor (λmax = 365 nm) at room temperature. Complete conversion is achieved after only 10 min of irradiation as measured by 1H NMR spectroscopy (Figure 6), suggesting successful formation of the highly active ruthenium-arene complex bearing an NHC ligand. In addition, polyNb [with a number-average molecular weight of 288 kDa and relatively narrow dispersity values (Ð = 1.5)] is obtained as determined by size exclusion chromatography.

Step 3.2 describes a miniemulsion photoROMP procedure. High conversions (70-80%) are achieved (Figure 7). As can be seen in Figure 8, the initial droplet size measured by DLS is 92 nm. The final particles exhibit a size of 102 nm (0.140) close to the initial droplet size. TEM observations show perfectly spherical particles with sizes in agreement with DLS data.

Figure 2: NMR monitoring of IMesH+BPh4- deprotonation and subsequent synthesis of IMes-CS2. 1H NMR spectra of IMesH+BPh4-/ITX (2/1 equiv.)mixturein THF-d8 (a) before UV exposure and (b) after 10 min irradiation at 365 nm (0.12 mW cm-2) in a photochemical reactor; shown are (c) 13C NMR spectra in DMSO-d6 of the precipitate recovered after addition of CS2. This figure has been modified from a previous publication9. Please click here to view a larger version of this figure.

Discussion

Reported here is an easy and versatile protocol for the in-situ generation of NHC upon UV-irradiation at 365 nm. The anion exchange reaction between 1,3-dimesitylimidazolium chloride and sodium tetraphenylborate provides straightforward access to the NHC protected from IMesH+BPh4- in quantitative yield. Nevertheless, if using another starting imidazolium salt, the solvent employed to perform the metathesis reaction should be chosen with care so that it allows the solubilization of both starting salts (imidazolium salts and sodium tetraphenylborate) and the precipitation of the imidazolium tetraphenylborate product. As such, ethanol is often the most appropriate solvent to perform this reaction.

The photogeneration of the NHC IMes by irradiation at 365 nm of the 2 components system IMesH+BPh4-/ITX can produce NHC yields up to 50%, but lower yields can be obtained depending on the experimental conditions employed. In particular, the use of solvents containing water or protic species favors secondary reactions such as the deprotonation of these protic species by BPh4- and/or the reprotonation of IMes, decreasing the overall yield of released IMes. Indeed, NHC are known to be sensitive to water and other impurity traces, so it is recommended to use dried solvents when attempting to photogenerate the NHC IMes. Despite their water/protic sensitivity, NHCs are much more reactive towards metallic substrates such as [RuCl2(p-cymene)]2, which allows for the ROMP of Nb to be performed in miniemulsion. It has been noticed that the presence of dioxygen can also alter the course of the reaction. Indeed, dioxygen is known to react with ITX triplet, preventing the release of IMes. Because an electron transfer is involved during the generation of NHC, the reaction is also assumed to be highly dependent on solvent polarity. Finally, when attempting to photogenerate IMes from ITX/IMesH+BPH4- in a reaction media, the latter should be chosen to provide good solubilization of the IMesH+BPh4- salt and no absorption of UV light up to 350 nm.

As opposed to other methods that rely on temperature, dilution, or pH changes to generate in situ NHC, this approach involves radiation as the external stimulus, with a distinctive advantage being spatial/temporal control of the reaction. Thanks to manifold polymerization reactions catalyzed/initiated by NHC, we envision that a photolatent NHC can foster new photopolymerization reactions such as photoROMP as detailed in this study. In addition, because NHCs are well-established stabilizing ligands, we believe that the photochemical preparation of organometallic complexes may benefit from this photogenerating NHC system. Finally, because NHCs are employed as reactants or catalysts in many organic chemistry reactions, their photogeneration should be of interest to chemists who wish to involve NHCs in cascade reactions at specific times.

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Disclosures

The authors have nothing to disclose.

Acknowledgments

Financial support by the French National Research Agency (ANR program: DS0304 2016, contract number: ANR-16-CE07-0016) and the French Ministry of Research (doctoral grant of Emeline Placet) are gratefully acknowledged.